1. Field of the Invention
The present invention generally relates to logically partitioned computer systems and more particularly to maintaining virtualized real time clocks based on a hardware real time clock.
2. Description of the Related Art
In a computing environment, parallel processing generally refers to performing multiple computing tasks in parallel. Traditionally, parallel processing required multiple computer systems, with the resources of each computer system dedicated to a specific task, or allocated to perform a portion of a common task. However, recent advances in computer hardware and software technologies have resulted in single computer systems capable of highly complex parallel processing, by logically partitioning the system resources to different tasks. In a logically partitioned computer system, available system resources are allocated among multiple logical partitions, each designed to appear to operate independently of the other. Management of the allocation of resources among logical partitions is typically accomplished via a layer of software components, commonly referred to as a partition manager.
An objective of the partition manager is to allow each logical partition to independently run software (e.g., operating systems and operating system-specific applications), typically developed to run on a dedicated computer system, with little or no modification. For example, one logical partition may be running a first operating system, such as IBM's OS/400, a second logical partition may be running a second operating system, such as IBM's AIX, while a third logical partition may be running a third operating system, such as Linux. By providing the ability to run multiple operating systems on the same computer system, a logically partitioned system may provide a user with a greater degree of freedom in choosing application programs best suited to the user's needs with little or no regard to the operating system for which an application program was written.
The partition manager typically accomplishes the objective of allowing each of the logical partitions to independently run software by presenting each logical partition with a set of virtual resources (software components) that operate, from the perspective of the logical partition, in an identical manner to corresponding hardware components. In other words, the partition manager may allow each logical partition to, in affect, operate as an independent virtual computer system (or virtual machine) with its own set of virtual resources.
One example of a virtual resource that may be provided to each logical partition is a virtual real time clock, which may be derived from a single actual hardware real time clock that is battery backed to allow for system time keeping in the absence of power. The partition manager typically keeps track of the virtualized real time clocks by maintaining offsets, for each logical partition, from the hardware real time clock. While the logical partitions are able to modify the virtual real time clocks, they are typically prohibited from modifying the hardware real time clock, which might render the offsets invalid.
While each virtual real time clock may be set to a different time (e.g., via a different partition offset), it is a general objective for the serviceability of a logically partitioned system to synchronize the real time clock among various components responsible for maintaining the partitions in the system, which may include the partition manager, a service processor, a remote console used for controlling the system as a whole, or any combination of such components. It is furthermore an objective for such systems to have the capability to perform this synchronization without the restriction that the partition manager be powered up and fully operational for this synchronization to occur. Therefore, it may be permissible to make changes to the hardware real time clock while the partition manager is not running.
However, if the hardware real time clock is changed while the partition manager is not running, the virtual real time clock data (e.g., offsets) for the logical partitions may no longer be valid. Accordingly, in conventional logically partitioned systems, changes to a hardware real time clock while the partition manager is not running may result in the loss of all partition virtual real time clocks, which must then be reset by the user, one by one, which may require significant time and effort. In an effort to prevent this scenario on conventional logically partitioned systems, access to setting the hardware real time clock while the partition manager is not running is severely restricted, which may undesirably limit serviceability.
Accordingly, there is a need for an improved method and apparatus for maintaining virtual real time clocks, based on a hardware real time clock, that allows changes to the hardware real time clock while logical partitioning software is not running.